The present invention relates generally to the field of waveguides and, more particularly, to waveguide filter elements and still more specifically to techniques for varying the cut-off frequency of waveguides particularly at the EHF band.
Conventional waveguide filters use elements that are in electrical and mechanical contact with the waveguide walls. Typical examples of these types of filters include inductive posts and inductive irises. These reactive elements are realized by means of metal rods or plates that are inserted into carefully machined openings and bonded to the walls of the waveguide by means of soldering, welding or compression techniques. Newer printed circuit waveguide filters also use such elements that are printed on substrates that are held suspended between the waveguide walls with firm metallic contacts at the walls. These filters, known as fin-line filters are simpler to make than irises and inductive posts but also require very precise machining to split the waveguide and cut the groove for supporting the substrate. Because the foregoing described type of filter elements are in contact with the waveguide walls and because currents flow in the junctions between the elements and the waveguide walls, and because of junction imperfections, the filter loss and reflection quality are often degraded.
Regardless of the element implementation technique used in prior art printed circuit filter elements, the inductive stub elements and the iris reactances both require firm contact to the waveguide walls. It is therefore nearly impossible to simultaneously vary the reactances of the filter elements. For this reason, variable filters for the EHF band are not available.
Waveguide phase shifters have been disclosed as, for instance, in Microwave Transmission Circuits, edited by George L. Ragan, p.p. 513-516 (1948). Such phase shifters utilize some form of screw tuning mechanisms such as sidewall screw tuners to vary the position of a long dielectric slab that extends longitudinally down the waveguide. The position of the slab may be changed laterally across the interior of the guide by utilizing the sidewall screw tuning mechanism. While such devices have been utilized in the past to cause a phase shift in the signal propagating through the section of the waveguide in which the phase shifter is utilized, it has never been suggested to utilize the screw tuning mechanism in combination with a printed circuit filter for the purpose of selectively adjusting the waveguide cut-off frequency as in the present invention.